This invention relates generally to paper machines.
More particularly, the present invention relates to a method for calendering paper webs which employs a plurality of hard or metal rolls and a plurality of soft or filled rolls which are situated to define soft supercalender nips with the hard rolls.
Further, the present invention relates to apparatus for calendering paper webs adapted to be directly associated with a paper machine or the like and to operate as a so-called machine supercalender and which comprises an upright series of hard calender rolls constituting a roll stack and a plurality of soft rolls situated with respect to said hard rolls to define soft supercalender nips with the hard rolls.
The paper web leaving the drying section of a paper machine is generally not suitable for sale as such in that additional finishing operations are still required. One of such finishing operations comprises calendering of the paper web by which the smoothness and gloss or finish of the paper is obtained as is its final desired density. Calendering is accomplished by conducting the continuous paper web through press nips defined between calender rolls as is well known.
Conventionally, calendering is effected through the use of a so-called "machine calender" which is directly associated with the paper machine. It is also well known that the calendering treatment may be supplemented when desired by supercalendering the web in a separate so-called "supercalender".
Such calendering machines in general are constituted by rolls which may be hard rolls or soft rolls. In this connection, as used in the present context, hard rolls will be understood as referring to rolls having a hard, smooth surface formed of a material such as chill-cast iron, steel or the like. In the present context soft rolls will be understood as referring to rolls whose surface layer comprises a resilient, elastic, non-metallic material. In this connection, soft rolls generally comprise so-called filled rolls wherein the resilient material forming the same comprises paper sheets which have been assembled on the core or shaft of the roll at right angles thereto and which have been compressed under large forces to form a coherent, compact roll covering.
The rolls constituting a typical machine calender generally are all hard rolls. On the other hand, soft rolls are used in addition to hard rolls in the calender stacks of supercalenders. Thus, in conventional, well-known supercalenders, hare and soft rolls alternate with each other in the calender stack so that the number of soft rolls is generally substantially the same as the number of hard rolls.
Further as used herein, the term "soft nip" shall be understood as referring in a calender to the line of contact between a soft roll and a hard roll located in nip defining relationship with each other. A "hard nip" will be understood as referring to the line of contact defined between two hard rolls which are in nip defining relationship in the calender stack.
The term "nip" will be understood as being used in its broad sense, i.e., referring not only to the line of contact between two rolls but, additionally, as referring to that region of a roll or rolls where a nip can be established. The operation of separating two rolls which are in mutual nip defining relationship will be referred to as "opening" a nip while the term "closing" a nip will be understood as referring to the step of moving two rolls which are initially separated from each other into nip defining relationship with each other.
Furthermore, as used in the present context the terms "machine calendering" and "machine burnishing" refer to the treatment of a web which is effected exclusively as the same passes through a hard calender nip as a result of which the web is compacted and its surface smoothed so as to obtain a so-called "machine finish". Likewise, the terms "supercalendering" or "super burnishing" will be understood as meaning the web treatment which is effected in soft calender nips and as a result of which a gloss is imparted to the web surface which is substantially superior to the machine finish obtained in hard calender nips. In this connection, it is understood that hard nips may also be found in supercalenders. However, in the context of the present invention, the term "super gloss", rather than indicating the degree of glossiness imparted to a paper web, refers to the fact that the gloss of the web surface has been produced at least a part by a supercalendering process in soft nips. The degree of super gloss accordingly may vary depending, for example, on the number of soft nips utilized in the supercalendering process, on the nip pressure, etc.
Depending on the type of paper which is being treated and on the requirements for the finished product, machine calendering may be accomplished utilizing only a single nip calender, i.e., a calender defined by a single pair of rolls. However, conventionally, a machine calender stack will comprise from six to eight rolls which correspondingly define between five and seven nips.
Generally, the object of the supercalendering process is to obtain an equal gloss on both surfaces of the paper. For this reason, the paper web is generally arranged to pass through the soft nips in a manner such that both surfaces of the web will alternately face a hard roll whose hard, smooth surface will act to produce a gloss to a greater extent than the surface of a soft roll.
In any event, two soft nips will not provide a super gloss to the paper web which would be substantially superior to a machine finish. For this reason, the number of soft nip pairs in supercalendering is usually greater than one and in practice separate supercalender stacks may be utilized having up to ten nip pairs.
In an effort to increase production in paper machines, attempts have been made to design calenders which combine the functions of both machine calenders and supercalenders. For example, applicant's U.S. Pat. No. 4,128,053 discloses a unitary machine-supercalender which is adapted to be directly associated with a paper machine and which is constituted by a conventional roll stack defined by a series of hard rolls and wherein a substantially equal number of soft rolls situated outside of the roll stack in nip defining relationship with the hard rolls define soft nips therewith.
With the above-mentioned combined machine calender and supercalender, it is possible to effect a supercalendering of the web in a desired manner immediately upon the web leaving the paper machine without the need for any intermediate operations. However, it has been found that in certain situations the super gloss imparted to the paper web by the calendering treatment effected by such apparatus is not entirely satisfactory in that the gloss is spotty or mottled rather than being uniform, i.e., localized areas of the surface of the paper web have a higher gloss than other areas. Furthermore, it has been found that a web being subjected to treatment in such a machine-supercalender has a tendency to blacken at localized regions due in part to the drawback that the hard nips found in such apparatus are unduly hard and inelastic with respect to the paper being produced.